1. Field of the Invention
The present invention relates to a Code Division Multiple Access (CDMA) multi-code transmitter for summing a plurality of baseband signals having spread spectra with respective spreading codes and transmitting the summed baseband signals. Particularly, the present invention relates to a reduction of power consumption of a transmitting amplifier of the transmitter in transmitting the signals.
2. Description of Related Art
In the CDMA system, the discrimination of individual channels is realized by not dividing frequency or time but using different spreading codes having small mutual correlations. The channels discriminated by the spreading codes will be referred to as "code channels" hereinafter. Therefore, in the CDMA system, it is possible to transmit signals having spectra spread with a plurality of mutually different spreading codes, that is, multi-code, by using one and the same transmitter in one and the same frequency band at the same time.
FIG. 1 is a block diagram showing a construction of a conventional CDMA multi-code transmitter. In the shown conventional CDMA multi-code transmitter, the spectra of data signals on the respective code channels are spread with different spreading codes and summed simply, as shown in FIG. 1. A transmitting method in a usual CDMA system will be described with reference to FIG. 1. In this description, it should be noted that the Quadrature Phase Shift Keying (Q-PSK) system is used as a modulation system for transmission.
The data signals 0.about.n of the respective channels are input to Serial-to-Parallel (S/P) converters 1-0.about.1-n in which data of the data signals are converted into I channel data and Q channel data, respectively. Describing the i-th channel (i=0.about.n), for example, data DIi and DQi converted by the S/P converter 1-i are input to respective multipliers 2-1i and 2-2i. The multipliers 2-1i and 2-2i multiply the data DIi and DQi with spreading code PNi assigned to the i-th code channel and generated by a spreading code generator 3-i to spread the spectrum. The signal whose spectrum is spread by the multiplier 2-1i is band-limited by a band-pass filter 4-1i and input to an adder 6-1. The signal whose spectrum is spread by the multiplier 2-2i is band-limited by a band-pass filter 4-2i and input to an adder 6-2. The adder 6-1 sums the signals I0.about.In passed through the band-pass filters 4-10.about.4-1n and outputs a modulated signal I. The adder 6-2 sums the signals Q0.about.Qn passed through the band-pass filters 4-20.about.4-2n and outputs a modulated signal Q. The modulated signals I and Q are sent to a transmitting portion 7 and a carrier signal is quadrature-modulated with these signals, amplified by a transmitting amplifier of the transmitting portion 7 and transmitted from the antenna as electromagnetic wave.
FIG. 2 shows an example of the eye-pattern in the conventional system, with reference to which an increase of peak value produced by the synthesis performed by the adder will be described. It should be noted that, for simplicity of description, a result of synthesis of the signals I of two code channels is shown in FIG. 2. Further, it is assumed that the band-pass filters are Nyquist filters, respectively.
In FIG. 2, the sum of the signals I1 and I2 on the different code channels is the I signal and peak values VP1 and VP2 of the signals I1 and I2 appear at intermediate positions between adjacent sample points, respectively. Therefore, a peak value VP of the signal I resulting from a synthesis of two signals I1 and 12 is represented by the following equation: EQU VP=VP1+VP2 (1)
Therefore, if VP1 and VP2 are equal, VP becomes twice VP1. On the other hand, since a mean power value of the signal I becomes only a square root of 2, a peak factor is increased by 3 dB.
In order to solve this problem, it has been proposed to shift times at which peak values of a plurality of signals to be summed are produced. In, for example, Japanese Patent Application Laid-open No. Hei 6-244821, a pseudo random code is commonly given to all communication stations and a plurality of transmitters transmit signals to receivers by synchronizing transmitting operations of the transmitters with time differences given to the pseudo random code such that peaks of correlation values on receivers corresponding to the respective transmitters neither interfere with others nor are interfered by others.
In Japanese Patent Application Laid-open No. Hei 7-50649, the utilization efficiency of frequency is improved by assigning a series of spreading codes having small mutual correlation to a plurality of subscribers. On a transmitting side, two series of spreading codes with a time space of 1 chip between them, each of which has a self correlation function having peak value every two spreading code times (chips) and zero value at intermediate time between adjacent peak values, are produced and information codes in respective channels are spectrum-spread with these series of spreading codes and transmitted by summing the spectrum-spread information codes.
However, in each of these conventional CDMA multi-code transmitters, there is a problem that the efficiency of a transmitting amplifier is degraded and power consumption is increased. The reason for this resides in the peak factor of signal. The peak factor is a ratio of a peak level of an input signal to the transmitting amplifier to a mean level of the signal. When this ratio is large, the transmitting amplifier must maintain a linearity with respect to peak value which is large compared with the mean level and, therefore, it is necessary to have an enough back-off (ratio of a saturation level of an amplifier to a mean transmitting level thereof: represented by dB) large in an output level of the amplifier. However, when the back-off is made large, the efficiency of the amplifier is degraded. Therefore, power consumption of the device in increased.
In either of the above mentioned prior arts, the frequency utilizing efficiency is improved by assigning one and the same spreading code to a plurality of channels so that a plurality of users can utilize the spreading code, and, in order to facilitate a separation and demodulation of one of the channels by using one and the same spreading code on a receiving side, it is necessary to make the peak factor large enough to easily identify the peak value. Therefore, the above mentioned problem is indispensable.
An object of the present invention is to provide a multi-code transmitter of CDMA system, which is capable of solving the problems inherent to the prior art, by using mutually different spreading codes to make peak values of the eye-pattern smaller to thereby reduce power consumption of the transmitter.